Rapid sulfur melt diffusion into carbon host for making electrodes

ABSTRACT

A free-standing electrically conductive porous structure suitable to be used as a cathode of a battery, including an electrically conductive porous substrate with sulfur diffused into the electrically conductive porous substrate to create a substantially uniform layer of sulfur on a surface of the electrically conductive porous substrate. The free-standing electrically conductive porous structure has a high performance when used in a rechargeable battery. A method of manufacturing the electrically conductive porous structure is also provided.

This invention was made with government support under Grant No.NSF-CBET-1236466 awarded by the U.S. National Science Foundation. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to electrodes for lithium batteries. Inparticular, the invention relates to these electrodes and to a method ofmanufacturing these electrodes for lithium batteries by rapidlydiffusing sulfur into a porous conductive substrate.

2. Description of the Related Technology

Since the commercialization of lithium-ion batteries in the early1990's, demand of rechargeable batteries for portable electronics,electric vehicles, and large-scale electricity storage has exploded.Lithium sulfur (Li—S) batteries have attracted attention owing to itshigh theoretical capacity of 1675 mAh g⁻¹ and its high energy density(˜2500 Wh kg⁻¹) which is about 5 times greater than that of lithium-ionbatteries (about 500 Wh kg⁻¹). See L. F. Nazar, M. Cuisinier and Q.Pang, MRS Bulletin, 2014, 39, 436-442; S. E. Cheon, K. S. Ko, J. H. Cho,S. W. Kim, E. Y. Chin and H. T. Kim, Journal of The ElectrochemicalSociety, 2003, 150, A796-A799; and US2002/0045102. In addition, sulfuris earth-abundant, inexpensive, and environmentally benign in comparisonwith other materials such as ion.

There are three key challenges for commercialization of Li—S batteries:(i) the electronically insulating nature of sulfur (5×10⁻³⁰ S cm⁻¹ at25° C.), (ii) large volume changes during lithiation-delithiation cycles(about 80%), and (iii) dissolution of soluble reaction intermediates(Li₂S_(n) ²⁻, 2<n≤8) into electrolytes which migrate to the anode uponcycling (the so-called shuttling effect). See D. W. Wang, Q. Zeng, G.Zhou, L. Yin, F. Li, H. M. Cheng, I. R. Gentle and G. Q. M. Lu, Journalof Materials Chemistry A, 2013, 1, 9382-9394; N. Jayaprakash, J. Shen,S. S. Moganty, A. Corona and L. A. Archer, Angewandte Chemie, 2011, 123,6026-6030; S. Evers and L. F. Nazar, Accounts of Chemical Research,2013, 46, 1135-1143; and M. S. Song, S. C. Han, H. S. Kim, J. H. Kim, K.T. Kim, Y. M. Kang, H. J. Ahn, S. X. Dou and J. Y. Lee, Journal of TheElectrochemical Society, 2004, 151, A791-A795. All of these challengescontribute to poor performance of Li—S batteries by causing low sulfurutilization, fast capacity fade, poor cycling stability and lowcoulombic efficiency. Thus, there are significant research efforts toimprove sulfur cathodes by adding conductive components to enhancesulfur utilization, using nanostructures to increase sulfur loadingwithin the cathode, and confining sulfur in the cathodes to reducepolysulfide shuttling.

These efforts yielded various products based on highly sophisticatedcomposites having different nano-architectures. For example, Liang et alproduced composites by reacting manganese dioxide nanosheets withsulfides yielding high capacity and long cycle life (X. Liang, C. Hart,Q. Pang, A. Garsuch, T. Weiss and L. F. Nazar, Nat Commun, 2015, 6). Cuiet al. produced sulfur-TiO₂ yolk-shell nanostructured cathodes with longcycle life. Many groups have developed micro/mesoporous carbon/sulfurcomposites that show excellent confinement of sulfur and solublepolysulfides. See K. Mi, Y. Jiang, J. Feng, Y. Qian and S. Xiong,Advanced Functional Materials, 2016, 26, 1571-1579; H. J. Peng, J. Q.Huang, M. Q. Zhao, Q. Zhang, X. B. Cheng, X. Y. Liu, W. Z. Qian and F.Wei, Advanced Functional Materials, 2014, 24, 2772-2781; X. a. Chen, Z.Xiao, X. Ning, Z. Liu, Z. Yang, C. Zou, S. Wang, X. Chen, Y. Chen and S.Huang, Advanced Energy Materials, 2014, 4, 1301988; Z. Li, J. T. Zhang,Y. M. Chen, J. Li and X. W. Lou, Nat Commun, 2015, 6; and J. G. Wang, K.Xie and B. Wei, Nano Energy, 2015, 15, 413-444. However, thesesophisticated sulfur-nanomaterial composites require harsh sulfur slurryprocessing for their production. Such processes typically compriserigorous mixing of sulfur/nanomaterials, inactive conductive carbonpowders (about 10-30 wt. %), and insulating binding agents (about 10 wt.%) in a highly toxic solvent (NMP) to generate a thick slurry which iscast onto a heavy current collector (typically aluminum foil, about 5 mgcm⁻²). See Y. Zhao, Y. Zhang, Z. Bakenova and Z. Bakenov, Frontiers inEnergy Research, 2015, 3 and G. Zhou, S. Pei, L. Li, D. W. Wang, S.Wang, K. Huang, L. C. Yin, F. Li and H. M. Cheng, Advanced Materials,2014, 26, 625-631. The additives and current collector account for30-50% of the electrode weight. The final cathodes (with the currentcollector) contain only about 25 wt. % sulfur (a very low sulfurloading) despite having started with a sulfur nanocomposite with 80 wt.% sulfur. Furthermore, it is unclear to what extent the originalarchitectures are retained after the rigorous mixing of the slurrycontaining the nanomaterials.

Other methods avoiding the slurry processing have been also beendeveloped for making free-standing cathodes, such as by using vacuumfiltration. See Y. Zhao, F. Yin, Y. Zhang, C. Zhang, A. Mentbayeva, N.Umirov, H. Xie and Z. Bakenov, Nanoscale Research Letters, 2015, 10,450; A. Schneider, C. Suchomski, H. Sommer, J. Janek and T. Brezesinski,Journal of Materials Chemistry A, 2015, 3, 20482-20486; Y. Chen, S. Lu,X. Wu and J. Liu, The Journal of Physical Chemistry C, 2015, 119,10288-10294; Z. Yuan, H. J. Peng, J. Q. Huang, X. Y. Liu, D. W. Wang, X.B. Cheng and Q. Zhang, Advanced Functional Materials, 2014, 24,6105-6112; C. Wu, L. Fu, J. Maier and Y. Yu, Journal of MaterialsChemistry A, 2015, 3, 9438-9445; L. Zhu, H. J. Peng, J. Liang, J. Q.Huang, C. M. Chen, X. Guo, W. Zhu, P. Li and Q. Zhang, Nano Energy,2015, 11, 746-755; and W. Ni, J. Cheng, X. Li, Q. Guan, G. Qu, Z. Wangand B. Wang, RSC Advances, 2016, 6, 9320-9327. Unfortunately, vacuumfiltration is impractical for large-scale production and requiresextended drying (about 24-60 hours) to remove residual solvent.

Another significant drawback for producing these complex sulfurnanocomposites is that it involves a time-consuming sulfur depositionstep. Regardless of the type of electrode, slurry-based orfree-standing, nearly all porous nanostructures require laborious sulfurmelt-diffusion, a technique where the materials are held at atemperature between the sulfur's melting (about 119° C.) and boiling(about 445° C.) temperatures for extended dwell times (10-12 hours) toallow liquid sulfur to diffuse into pores by capillary forces. See Z.Li, J. T. Zhang, Y. M. Chen, J. Li and X. W. Lou, Nat Commun, 2015, 6;J. G. Wang, K. Xie and B. Wei, Nano Energy, 2015, 15, 413-444; Y. Chen,S. Lu, X. Wu and J. Liu, The Journal of Physical Chemistry C, 2015, 119,10288-10294; C. Wu, L. Fu, J. Maier and Y. Yu, Journal of MaterialsChemistry A, 2015, 3, 9438-9445; X. Huang, B. Sun, K. Li, S. Chen and G.Wang, Journal of Materials Chemistry A, 2013, 1, 13484-13489; Z. Zhang,Q. Li, K. Zhang, W. Chen, Y. Lai and J. Li, Journal of Power Sources,2015, 290, 159-167; and L. Zeng, F. Pan, W. Li, Y. Jiang, X. Zhong andY. Yu, Nanoscale, 2014, 6, 9579-9587. For example, Xu et al fabricatedsulfur/porous graphitic carbon composites by melting and diffusingsulfur at 155° C. in 12 hours (G. L. Xu, Y. F. Xu, J. C. Fang, X. X.Peng, F. Fu, L. Huang, J. T. Li and S. G. Sun, ACS Applied Materials &Interfaces, 2013, 5, 10782-10793); He et al created bimodal mesoporouscarbon/sulfur by melting and diffusing sulfur at 155° C. overnight (G.He, X. Ji and L. Nazar, Energy & Environmental Science, 2011, 4,2878-2883); Lu et al produced sulfur/graphene oxide-ZnO by melting anddiffusing sulfur at 160° C. in 10 hours (M. Yu, A. Wang, F. Tian, H.Song, Y. Wang, C. Li, J. D. Hong and G. Shi, Nanoscale, 2015, 7,5292-5298); and Li et al made sulfur/mesoporous carbon composites usinga two-step melt-diffusion, 155° C. for 8 hours followed by 300° C. for 3hours (G. Li, H. Jing, H. Li, L. Liu, Y. Wang, C. Yuan, H. Jiang and L.Chen, Ionics, 2015, 21, 2161-2170).

When a shorter heating time for the sulfur was used in the abovemethods, the performance of the resulted electrodes is severely limited.For example, when 15 minutes heating time was used in some processes,the C-rates were generally low, not exceeding 0.06 C and 0.17 C, norcycling stability beyond 50 cycles. Thus, a simple and practical sulfurmelting-diffusion method that does not compromise electrochemicalperformance of the resulted electrodes is desired.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a free-standingelectrically conductive porous structure suitable to be used as acathode of a battery, comprising an electrically conductive poroussubstrate with a substantially uniform layer of sulfur on the surface ofthe electrically conductive porous substrate, where the free-standingelectrically conductive porous structure contains less than 10 wt. % ofgraphene, based on a total weight of the electrically conductive porousstructure, and a battery using the free-standing electrically conductiveporous structure as a cathode has a C rate of at least 0.2 C at adischarge capacity of from about 400 mAh g⁻¹ to about 1675 mAh g⁻¹ witha cycling stability of at least 100 cycles

In another aspect, the free-standing electrically conductive porousstructure may be suitable to be used as a cathode without physical orchemical activation.

In another aspect, in any of the foregoing embodiment, the graphene, ifpresent, is not functionalized or not functionalized with an amine.

In another aspect, the C rate of the battery using the free-standingelectrically conductive porous structure of any of the foregoingembodiments may be at least 0.5 C, at least 0.75 C, or at least 1 C.

In another aspect, the cycling stability of the battery using thefree-standing electrically conductive porous structure of any of theforegoing embodiments may be for at least 120 cycles, or at least 150cycles or at least 170 cycles.

In yet another aspect, the free-standing electrically conductive porousstructure of any one of the previous embodiments has a flexibility thatpasses the Mandrel Bend test (ASTM D522).

In yet another aspect, in the battery using the free-standingelectrically conductive porous structure of any one of the previousembodiments the discharge capacity may be in the range of from about 700mAh g⁻¹ to about 1625 mAh g⁻¹, or from about 800 mAh g⁻¹ to about 1600mAh g⁻¹, or from about 900 mAh g⁻¹ to about 1500 mAh g⁻¹ at 0.5 C ratewith a cycling stability of at least 100 cycles.

In yet another aspect, in the battery using the free-standingelectrically conductive porous structure of any one of the previousembodiments the discharge capacity may be in a range of from about 400mAh g⁻¹ to about 1675 mAh g⁻¹, or from about 900 mAh g⁻¹ to about 1600mAh g⁻¹, or from about 1100 mAh g⁻¹ to about 1500 mAh g⁻¹ at 0.2 C ratewith a cycling stability of at least 100 cycles.

In another aspect, the free-standing electrically conductive porousstructure of any one of the previous embodiments may comprise aconductive additive in the sulfur selected from the group consisting ofconductive carbon powders, graphite powders, mesoporous carbons,activated carbons, carbon nanotubes, MXenes, conductive polymers,conductive metal oxides/suboxides, metals and any other material thatconducts electrons.

In another aspect, the electrically conductive porous substrate of anyone of the previous embodiments may comprise a material selected fromcarbon nanofibers, carbon nanotubes, carbon rods, and combinationsthereof.

In another aspect, the electrically conductive porous substrate of anyone of the previous embodiments may have a porosity in a range of fromabout 10% to about 90%, or from about 70% to about 85%.

In another aspect, the electrically conductive porous substrate of anyone of the previous embodiments may have pores with average porediameter in a range of from about 0.1 nanometer to about 100 microns, orfrom about 1 nanometer to about 70 microns, or from about 100 nanometersto about 50 microns.

In another aspect, the electrically conductive porous substrate of anyone of the previous embodiments has a conductivity in a range of fromabout 10⁻³ S/cm to about 10⁵ S/cm, or from about 10⁻¹ to about 10³ S/cm,or from about 1 to about 10² S/cm.

In another aspect, the free-standing electrically conductive porousstructure of any one of the previous embodiments may have a sulfurcontent in a range of from about 10 wt. % to about 90 wt. %, or fromabout 30 wt. % to about 80 wt. %, or from about 30 wt. % to about 70 wt.%, or from about 40 wt. % to about 60 wt. %, or from about 45 wt. % toabout 55 wt. %, based on a total weight of the structure.

In yet another aspect, the sulfur in the electrically conductive porousstructure of any one of the previous embodiments may have an averageparticle size in a range of from 10 nm to 1000 nm, or from 20 nm to 1000nm, or from 50 nm to 1000 nm, or from 100 nm to 1000 nm, or from 200 nmto 1000 nm.

In yet another aspect, the present invention provides a cathodecomprising the electrically conductive porous structure of any one ofthe previous embodiments.

In yet another aspect, the cathode of the previous embodiments may havea sulfur loading in a range of from about 0.1 mg cm⁻² to about 15 mgcm⁻², or from about 0.5 mg cm⁻² to about 7 mg cm⁻², or from about 1 mgcm⁻² to about 5 mg cm⁻².

In yet another aspect, the present invention provides a battery havingthe cathode of any one of the previous embodiments.

In yet another aspect, the present invention provides a method ofmanufacturing an electrically conductive porous structure for a cathodeof a battery, including a step of heating sulfur on an electricallyconductive porous substrate to a temperature sufficient to melt thesulfur and allowing the melted sulfur to diffuse into the electricallyconductive porous substrate.

In yet another aspect, the method of any one of the previous embodimentsmay use the sulfur in a powder form, or particle form.

In yet another aspect, the method of any one of the previous embodimentsmay be carried out at a temperature in a range of from about 119° C. toabout 170° C., or from about 130° C. to about 160° C., or from about140° C. to about 160° C., or from about 150° C. to about 160° C.

In yet another aspect, the method of any one of the previous embodimentsmay carry out the heating step for a period of from about 3 to about 500minutes, or from about 5 to about 100 seconds, or from about 5 to about50 seconds, or from about 3 to about 30 seconds.

In yet another aspect, the method of any one of the previous embodimentsmay be carried out at a pressure in a range of from about 15 psi toabout 2000 psi, or from about 50 psi to about 2000 psi, or from about100 psi to about 1000 psi, or from about 150 psi to about 800 psi, orfrom about 150 psi to about 500 psi.

In yet another aspect, the method of any one of the previous embodimentsmay use sulfur having an average particle size smaller than about 10 μm,or smaller than about 5 μm, or smaller than about 2 μm, or smaller thanabout 1 μm, or smaller than about 800 nm, or smaller than about 500 nm,or smaller than about 300 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation of the process for infusing sulfurinto a carbon nanofiber (CNF) substrate.

FIG. 1B shows a coin cell battery assembly with the electrode (cathode)of the present invention.

FIG. 2 shows the cathode of the present invention before and after a5-second heat-treatment, respectively.

FIGS. 3A-3B show scanning electron microscope (SEM) images ofsulfur-infused CNF substrate on the side from which sulfur was infusedand from the side opposite of sulfur infusion, respectively.

FIG. 3C shows a cross-section of the cathode of the present invention.

FIG. 3D shows the cross-section of FIG. 3C with an energy-dispersiveX-ray spectroscopy (EDS) sulfur map.

FIG. 3E shows the cross-section of FIG. 3C with an EDS carbon map.

FIG. 4 shows a thermogravimetric analysis (TGA) of the pure sulfur andpure CNF substrate.

FIG. 5 shows X-ray diffraction (XRD) results of pure sulfur as purchased(pristine sulfur) and sulfur after heat treatment.

FIG. 6 shows cyclic voltammetry of the cathode of the invention over thefirst 5 cycles at a 0.05 mV s⁻¹ scan rate.

FIG. 7 shows charge-discharge curves of the cathode of the inventiontested at 0.1 C, 0.2 C, and 0.5 C rates.

FIG. 8 shows cyclability of the cathode of the invention at a C rate of0.1 C over 100 cycles.

FIG. 9 shows rate capability of the cathode of the invention at C ratesof 0.1 C, 0.2 C, 0.5 C and 1 C.

FIG. 10 shows the cyclability of the cathode of the invention at astandard 0.2 C rate and a 0.2 C rate with an initial conditioning cycleat 0.1 C.

FIG. 11 shows the cyclability of the cathode of the invention at astandard 0.5 C rate and a 0.5 C rate with an initial conditioning cycleat 0.2 C or initial conditioning cycles at 0.1 C and 0.2 C.

FIG. 12 shows the long term cycling capacity and effective capacity ofconditioned cathode of the invention at a 0.5 C rate.

FIG. 13 shows long term cycling effective discharge capacity accountingfor the current collector using cathodes disclosed in the literature,which require slurry-processing and long sulfur deposition period. Alldata were generated at 0.5 C rate.

FIGS. 14A-14C show deconvoluted X-ray photoelectron spectroscopy (XPS)spectra of C, N, and O respectively for the cathode of the presentinvention.

FIG. 15 shows the cycling performance of a standard cathode cellassembly and a reversed cathode cell assembly, respectively, at a 0.2 Crate.

FIG. 16A shows the SEM image of the cathode of present invention on theside from which sulfur was infused.

FIG. 16B shows and the corresponding sulfur EDS map of the cathode ofFIG. 16A.

FIG. 16C shows the corresponding carbon EDS map of the cathode of FIG.16A.

FIG. 17A shows the SEM image of the cathode on the side opposite of theside from which sulfur was infused.

FIG. 17B shows the corresponding sulfur EDS map of the cathode of FIG.17A.

FIG. 17C shows the corresponding carbon EDS map of the cathode of FIG.17A.

FIG. 18A shows the SEM image of the cathode on the side from whichsulfur was infused after 15 seconds heating.

FIG. 18B shows the corresponding sulfur EDS map of the cathode of FIG.18A.

FIG. 18C shows the corresponding carbon EDS map of the cathode of FIG.18A.

FIG. 19A shows the SEM image of the cathode on the side from whichsulfur was infused after 30 seconds heating.

FIG. 19B shows the corresponding sulfur EDS map of the cathode of FIG.19A.

FIG. 19C shows the corresponding carbon EDS map of the cathode of FIG.19A.

FIG. 20 shows cycling performance of the cathode of the invention aftera standard 5 seconds of heat treatment and after 15 seconds of heattreatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For illustrative purposes, the principles of the present disclosure aredescribed by referencing various exemplary embodiments. Although certainembodiments are specifically described herein, one of ordinary skill inthe art will readily recognize that the same principles are equallyapplicable to, and can be employed in other systems and methods.

It is to be understood that the disclosure is not limited in itsapplication to the details of any particular embodiment shown.Additionally, the terminology used herein is for the purpose ofdescription and not of limitation. Furthermore, although certain methodsare described with reference to steps that are presented herein in acertain order, in many instances, these steps may be performed in anyorder as may be appreciated by one skilled in the art; the novel methodsare therefore not limited to the particular arrangement of stepsdisclosed herein.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural references unless the context clearly dictatesotherwise. Furthermore, the terms “a” (or “an”), “one or more” and “atleast one” can be used interchangeably herein. The terms “comprising”,“including”, “having” and “constructed from” can also be usedinterchangeably.

The C-rate is a measure of the rate at which a battery is dischargedrelative to its maximum capacity. A 1 C rate means that the dischargecurrent will discharge the battery with 100% of its maximum capacity in1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to adischarge current of 100 Amps at a rate of 1 C. The C-rate can beexpressed in one of two ways. Thus, a C-rate of C/5 is equivalent to aC-rate of 0.2 C, both of which refer to discharge of the battery with100% of its maximum capacity in 5 hours. Similarly, a C-rate of C/2 and0.5 C both refer to discharge of the battery with 100% of its maximumcapacity in 2 hours. If a battery only delivers 50% of its maximumcapacity, then at a 0.5 C (C/2) rate it will actually complete adischarge in only an hour, even though the current is set for C/2.

The present invention provides a free-standing electrically conductiveporous structure suitable to be used as a cathode of a battery. Thestructure comprises an electrically conductive porous substrate with alayer of sulfur on the surface of the electrically conductive poroussubstrate, where the free-standing electrically conductive porousstructure contains less than 10 wt. % of graphene, based on a totalweight of the electrically conductive porous structure. The layer ofsulfur and optional additives is located on the outer surface of thesubstrate and this sulfur diffuses into the pores of the substrate andforms a layer on the surface of the pores of the substrate as well. Abattery using the free-standing electrically conductive porous structureas a cathode has a C rate of at least 0.2 C at a discharge capacity offrom about 400 mAh g⁻¹ to about 1675 mAh g⁻¹ with a cycling stability ofat least 100 cycles.

In some embodiments, the free-standing electrically conductive porousstructure has a flexibility that passes the Mandrel Bend test (ASTMD522). The Mandrel Bend test is a standard test of the flexibility of amaterial, which is performed using the method of ASTM D522 or D522M-17.The test result may be a pass/fail for the flexibility of the testedmaterial. The free-standing electrically conductive porous structure maybe tested using the Mandrel Bend test with a passing result, which meansthat the structure has sufficient flexibility to meet the requirementsof the Mandrel Bend test.

Unlike structures provided in the prior art, the free-standingelectrically conductive porous structure of the present invention may besuitable to be used as a cathode without physical or chemicalactivation. Typical structures of the prior art require chemical orphysical activation to be employed as a cathode, which activation mayrender those structures mechanically fragile and sufficiently inflexibleso as not to be able to pass the Mandel Bend test, for example.

In some embodiments, the free-standing electrically conductive porousstructure is an uncomplicated structure including only the substrate andsulfur, as well as possibly some minor amounts of impurities introducedwith the sulfur. In certain embodiments of the free-standingelectrically conductive porous structure contains less than 10 wt. %, orless than 5 wt. %, or less than 2 wt. % or less than 1 wt. % ofgraphene, based on a total weight of the electrically conductive porousstructure. In one embodiment, the electrically conductive porousstructure is completely free of graphene. The graphene in theelectrically conductive porous structure, if present, may be graphenethat is not functionalized, or graphene that is not functionalized withan amine.

The free-standing electrically conductive porous structure of thepresent invention can be used as cathode without having to add a currentcollector. Thus, the effective sulfur content of the cathode is higherthan in many sulfur-based cathodes of the prior art which, require anadditional current collector.

The free-standing electrically conductive porous structure of thepresent invention has a C rate of at least 0.2 C, or at least 0.5 C, orat least 0.75 C, or at least 1 C. Further, the free-standingelectrically conductive porous structure of the present invention has acycling stability of at least 100 cycles, or at least 120 cycles, or atleast 150 cycles or at least 170 cycles.

In some embodiments, a battery using the free-standing electricallyconductive porous structure as a cathode has a discharge capacity in arange of from about 400 mAh g-1 to about 1675 mAh g⁻¹, or from about 700mAh g⁻¹ to about 1625 mAh g⁻¹, or from about 800 mAh g⁻¹ to about 1600mAh g⁻¹, or from about 900 mAh g⁻¹ to about 1500 mAh g⁻¹ at a 0.5 C ratewith a cycling stability of at least 100 cycles.

In some embodiments, the battery using the free-standing electricallyconductive porous structure has a discharge capacity in a range of fromabout 400 mAh g⁻¹ to about 1675 mAh g⁻¹, or from about 900 mAh g⁻¹ toabout 1600 mAh g⁻¹, or from about 1100 mAh g⁻¹ to about 1500 mAh g⁻¹ ata 0.2 C rate with a cycling stability of at least 100 cycles.

In some embodiments, the free-standing electrically conductive porousstructure comprises a conductive additive which may be introduced byincluding the conductive additive in the sulfur that forms a layer onthe surface of the substrate. This additive may be selected from thegroup consisting of conductive carbon powders, graphite powders,mesoporous carbons, activated carbons, carbon nanotubes, MXenes,conductive polymers, conductive metal oxides/suboxides, metals and anyother material that conducts electrons.

In some embodiments, the free-standing electrically conductive porousstructure comprises a viscosity reducing additive (such as selenium,tellurium, bromine or iodine) mixed with the sulfur to be introduced bymelting and diffusing the sulfur into the substrate. The viscosityreducing additive can reduce the viscosity of the melted sulfur thusfacilitating diffusion of the sulfur into the substrate.

In some embodiments, the free-standing electrically conductive porousstructure comprises an additive mixed with the sulfur to be introducedby melting and diffusing the sulfur into the substrate that canfavorably interact with sulfur and intermediate sulfur species (duringdevice operation) such as polysulfides to prevent or reduce activematerial dissolution/loss into the electrolyte. The interaction can be,but not limited to, polar interaction, Lewis-acid base interaction, orvia formation of thiosulfates. Examples of additives include compoundscontaining polar elements such as oxygen or nitrogen (metaloxides/suboxides such as zinc oxide, polymers such as polyaniline, forexample), or compounds containing metals such as titanium or vanadiumthat can interact with sulfur via Lewis acid base interaction.

The foregoing additives may be used alone or in any combination witheach other. The total amount of additive may comprise from 0.5 wt. % to30 wt. %, or from 1 wt. % to 10 wt. %, based on the total weight ofsulfur and additives. The skilled however understands that the amount ofthese additives may be adjusted depending on their conductivity,interaction strength, and particle size (surface area). Thus, routinetesting provides guidance as to the amount of an additive to be used inthe present invention.

The substrate used in the present invention may be carbon nanofibers,carbon nanotubes, carbon rods, and combinations thereof. Thesesubstrates may have a porosity in a range of from about 10% to about90%, or from about 70% to about 85%, and have pores with average porediameter in a range of from about 0.1 nanometer to about 100 microns, orfrom about 1 nanometer to about 70 microns, or from about 100 nanometersto about 50 microns. The substrates may have a conductivity in a rangeof from about 10⁻³ S/cm to about 10⁵ S/cm, or from about 10⁻¹ to about10³ S/cm, or from about 1 to about 10² S/cm.

In some embodiments, the free-standing electrically conductive porousstructure has a sulfur content in a range of from about 10 wt. % toabout 90 wt. %, or from about 30 wt. % to about 80 wt. %, or from about30 wt. % to about 70 wt. %, or from about 40 wt. % to about 60 wt. %, orfrom about 45 wt. % to about 55 wt. %, based on a total weight of thestructure.

In some embodiments, the sulfur in the electrically conductive porousstructure may have an average particle size in a range of from 10 nm to1000 nm, or from 20 nm to 1000 nm, or from 50 nm to 1000 nm, or from 100nm to 1000 nm, or from 200 nm to 1000 nm.

In some other embodiments, the electrically conductive porous structurehas a sulfur loading of at least about 70 wt. %, or at least about 75wt. %, or at least about 80 wt. %, or at least about 85 wt. %, of thetotal weight of the structure.

In some embodiments, the electrically conductive porous structure has asulfur loading of at least about 6 mg cm⁻², or at least about 6.5 mgcm⁻², or at least about 7 mg cm⁻², or at least about 7.5 mg cm⁻², or atleast about 8 mg cm⁻², or at least about 8.5 mg cm⁻², or at least about9 mg cm⁻².

In another aspect, the present invention provides a cathode comprisingthe electrically conductive porous structure. In some embodiments, thecathode has surface functional groups such as amine, hydroxyl andcarboxyl groups. The cathode does not contain a current collector. Thecathode may have a sulfur loading in a range of from about 0.1 mg cm⁻²to about 15 mg cm⁻², or from about 0.5 mg cm⁻² to about 7 mg cm⁻², orfrom about 1 mg cm⁻² to about 5 mg cm⁻². In yet another aspect, thepresent invention provides a battery including the cathode.

In another aspect, the present invention provides a method for producingan electrically conductive porous structure for use in cathodes ofbatteries. The method includes a step of heating sulfur on anelectrically conductive porous substrate to a temperature sufficient tomelt the sulfur and allowing the sulfur to diffuse into the electricallyconductive porous substrate.

In some embodiments, the electrically conductive porous substratecontains only electrically conducting materials that are stable in abattery electrolyte. Such materials include, but are not limited to,copper, tantalum, porous carbon and carbon fibers. In one aspect, theelectrically conductive porous substrate comprises carbon nanofibers,carbon nanotubes, carbon rods, or combinations thereof.

The electrically conductive porous substrate may have a porositysufficient to allow melted sulfur to diffuse into the substrate. Theporosity of the electrically conductive substrate may be in a range offrom about 50% to 90%, and is preferably in a range of about 70% to 85%with average pore sizes ranging from about 0.1 nanometers to about 100microns in diameter, preferably from about 1 nanometer to about 70microns in diameter, and most preferably from about 100 nanometers toabout 50 microns in diameter. The electrically conductive poroussubstrate may have pore sizes in a range of 0.1 nanometer to 100 micronsin diameter.

The conductivity of the electrically conductive porous substrate mayvary over a range of from about 10⁻³ S/cm to about 10⁵ S/cm, preferablyfrom about 10⁻¹ S/cm to about 10³ S/cm, or most preferably from about 1S/cm to about 10² S/cm.

The sulfur content of the electrically conductive porous structure maybe from about 10 wt. % to about 100 wt. %, depending on the specificapplication for which the electrode is designed. In some embodiments,the sulfur content of the electrically conductive porous structure ispreferably in a range of from about 20 wt. % to about 90 wt. %, orpreferably from about 30 wt. % to about 80 wt. %, or preferably fromabout 30 wt. % to about 70 wt. %, or preferably from about 40 wt. % toabout 60 wt. %, or preferably from about 45 wt. % to about 55 wt. %, allbased on a total weight of the electrically conductive porous structure.

In some embodiments, the sulfur used in the method for producing anelectrically conductive porous structure may be in the form of smallparticles, preferably nano-sulfur to improve sulfur utilization andaccessibility. The average particle size of the sulfur may be smallerthan about 10 μm, or smaller than about 5 μm, or smaller than about 2μm, or smaller than about 1 μm, or smaller than about 800 nm, or smallerthan about 500 nm, or smaller than about 300 nm. Without wishing to bebound by theory, the use of smaller sulfur particles reduce timerequired to melt the sulfur thus accelerating the sulfur diffusionprocess.

The sulfur used in the method may be in a powder form, or particle form.The sulfur may contain some impurities. The sulfur may be precipitatedsulfur, sulfur produced using any method or any other type ofcommercially available sulfur. Impurities in the sulfur normally do nothave a significant impact on the electrode performance. The size of thesulfur particles on the electrically conductive substrate at the startof the heating step may influence the parameters of the heating step.For example, the required heating time and/or temperature may beshorter/lower when employing nano-sulfur, as compared to largerparticles of other types of commercially available sulfur.

In the method, additional functional components may be blended with thesulfur prior to melting and/or diffusing the sulfur into the substrate.One such functional component is a conductive additive such as carbonblack, carbon nanotubes, activated carbon, mesoporous carbon, graphitepowder, MXenes, conductive polymers, metal oxides, and conductivesuboxides. For example, conductive carbon powder can be blended into thesulfur to introduce more conductivity and improve interfaces between thesulfur and electron-transport surfaces.

In some embodiments, conductive additives may be added for the purposeof improving the utilization of the sulfur.

In some embodiments, a viscosity reducing additive may be added to thesulfur prior to melting and/or diffusing the sulfur into the substrate.The viscosity reducing addition can reduce the viscosity of the meltedsulfur thus facilitating diffusion of the sulfur into the substrate.

In some embodiments, a polar additive may be added to the sulfur. Thepolar additive can interact with the sulfur in the free-standingelectrically conductive porous structure.

The foregoing additives may be used alone or in combination. The totalamount of additive(s) may be from 0.5% to 30%, or from 1% to 10%, basedon the total weight of sulfur and additives.

The sulfur may be heated to any temperature within the meltingtemperature range of sulfur, namely, from about 119° C. to about 170° C.The ultimate temperature of the sulfur may, depend on the heating time.In some embodiments, heating may be a rapid process, and the temperatureused to melt the sulfur is preferably in the range of from about 130° C.to about 160° C., or preferably from about 140° C. to about 160° C., ormore preferably from about 150° C. to about 160° C. In otherembodiments, the heating process may be a slower process and thetemperature used to melt the sulfur may be lower, such as in the rangeof from about 120° C. to about 150° C., or preferably from about 125° C.to about 140° C., or more preferably from about 130° C. to about 140° C.

Heating and diffusion of the sulfur into the electrically conductivesubstrate may take from as little as 3 seconds, to as long as about 500seconds. The time needed is partially dependent on the size and/orsurface area of the sulfur particles, as well as the pore size and/orthe thickness of the substrate. Larger pore sizes in the substrate allowmore rapid diffusion of sulfur into the substrate, and, as a result, theheating time may be shorter. Thicker substrates may require a longertime for the sulfur to diffuse throughout the substrate, and thus theheating time may need to be longer. In some embodiments, the time forheating and diffusion of the sulfur into the electrically conductivesubstrate may be in a range of from about 5 seconds to about 100seconds, or from about 5 seconds to about 50 seconds, or from about 5seconds to about 30 seconds.

The heating time and temperature are inversely correlated. For example,when the temperature is higher, the heating time is reduced. When thetemperature is lower, the heating time is increased. In one embodiment,the heating temperature is 120° C., and sulfur is melted and diffusedinto electrically conductive carbon nanofibers in about 60 seconds.

In some embodiments, the electrically conductive porous substrate may beexposed to a pressure in a range of from about 15 psi to about 2000 psiduring heating and diffusion of the sulfur into the substrate. Thedesired pressure is dependent on the pore size, porosity, and thicknessof the substrate. Preferably, the pressure is in a range of from about15 psi to about 2000 psi or from about 50 psi to about 2000 psi, or fromabout 100 psi to about 1000 psi, or from about 150 psi to about 800 psi,or from about 150 psi to about 500 psi. Elevated pressure can be used toreduce diffusion time and ensure diffusion of the sulfur throughout theelectrically conductive substrate.

In one embodiment, the electrically conductive porous substrate includeselectrospun carbon nanofibers, preferably in the form of a nanofibermat. The electrospun carbon nanofibers are impregnated with the meltedsulfur to produce an electrode that may be used in rechargeablebatteries such as lithium-sulfur and sodium-sulfur batteries. In thisembodiment, powdered sulfur is located on the carbon nanofiber mat andheated under pressure for a time of about 3-6 seconds, or about 4-5seconds to rapidly melt and diffuse the sulfur into the nanofiber mat.This produces a device-ready electrode that is free of binder andadditional or separate current collectors.

In one aspect, free-standing electrospun carbon nanofibers are producedby electrospinning a solution of polyacrylonitrile. The electrospunfibers are subsequently heated to carbonize the fibers thereby forming astable CNF mat. Sulfur is integrated into the CNF mat by rapidly heatingand melting sulfur on the CNF mat under applied pressure. This quickmelting-technique produces the final electrode, free of binding agentsand completely device-ready. As such, no vacuum filtration or otherprocessing is required.

The produced electrodes may be used to produce sulfur-containingcathodes for batteries. The resultant cathodes may have a sulfur loadingin a range of from about 0.1 mg cm⁻² to about 15 mg cm⁻², or from about0.5 mg cm⁻² to about 7 mg cm⁻², or from about 1 mg cm⁻² to about 5 mgcm⁻².

The cathodes may have an initial discharge capacity in a range of fromabout 700 mAh g⁻¹ to about 1675 mAh g⁻¹, or from about 900 mAh g⁻¹ toabout 1600 mAh g⁻¹, or from about 1100 mAh g⁻¹ to about 1500 mAh g⁻¹ ata C/10 rate.

The cathodes typically have an initial discharge capacity in a range offrom about 500 mAh g⁻¹ to about 1675 mAh g⁻¹, or from about 700 mAh g⁻¹to about 1625 mAh g⁻¹, or from about 800 mAh g⁻¹ to about 1600 mAh g⁻¹,or from about 900 mAh g⁻¹ to about 1500 mAh g⁻¹ at a C/5 rate.

The cathodes were used as electrodes in CR2032 coin cells forelectrochemical evaluation. Cathodes with a sulfur loading of 1.0 mgcm⁻² exhibited high discharge capacities of 1382, 1310, and 1140 mAh g⁻¹at C/10, C/5, and C/2 rates, respectively (1 C=1675 mAh g⁻¹). These highdischarge capacities indicate the CNF structure facilitates high sulfurutilization.

The current to each cell changes slightly because it must be calculatedfor the exact amount of sulfur loading in the individual electrode usedin the cell. If 1 C=1675 mAh g⁻¹ for sulfur and the cathode contains 1mg of sulfur, a 1 hr (or 1 C) charging rate means using a current of1.675 mAmps. For a cell with a cathode containing 1.1 mg of sulfur, a 1hr (or 1 C) charging rate would require 1.843 mAmps of current.

One cathode with a 0.6 mg cm⁻² (about 25 wt. %) sulfur loading tested ata C/10 rate exhibited a very high initial discharge capacity of 1625 mAhg⁻¹ (97% of the theoretical maximum, 1675 mAh g⁻¹) and a dischargecapacity of 1000 mAh g⁻¹ at 100 cycles. The initial discharge capacityof this cell relative to the weight of the entire electrode (activesulfur+inactive materials) was 406 mAh g⁻¹. Cathodes with a highersulfur loading (about 1 mg cm⁻² and 50 wt. %), tested at a faster C/5rate also showed initial discharge capacities greater than 400 mAh g⁻¹.These initial discharge capacities are well above that of a typicalLi-ion. For comparison, the maximum achievable discharge capacity of aLiCoO₂ cathode is about 140 mAh g⁻¹, which on a per gram-electrodebasis, reduces to approximately 98 mAh g⁻¹ when the currentcollector/binder weight (about 30-50% of total electrode weight in thiscase) is factored into the performance per unit weight of the electrode.

The battery with the cathode may be conditioned by running the batteryat a C-rate of from C/20 to C/10 for one cycle. Alternatively, thebattery may be conditioned by running the battery for: (i) one cycle atC-rate of from C/10 to C/5; (ii) one cycle at a C-rate of from C/5 toC/2; and (iii) one cycle at a C-rate of from C/2 to C/1.

The present invention provides a novel sulfur melting technique tosulfurize free-standing, conductive nanostructures such as electrospuncarbon nanofibers (CNFs). The nanostructure with the sulfur diffusedtherein can be used directly as facile cathodes for Li—S batterieswithout physical or chemical activation, or an additional or separatecurrent collector. The nanostructure has a large internal spacing,facilitating rapid sulfur diffusion under rapid heating and slightpressure (140° C. and <250 psi) to obtain sulfur-based cathodes with ahigh sulfur loading. Unlike slurry-based cathodes, the continuousinternal structure in the nanostructures provides the ability to retainuninterrupted electron transport networks by incorporating sulfur into abinder-free carbon host. Given the nanostructure-conductivity and thewell-distributed sulfur, the sulfur-based cathodes of the presentinvention exhibited excellent cycling performance at reasonable C-rates.High discharge capacities of 1153 mAh g⁻¹ and 1022 mAh g⁻¹ were achievedfor 0.1 C and 0.2 C rates, respectively. After conditioning, thesulfur-based cathodes of the present invention exhibited a dischargecapacity of 550 mAh g⁻¹100% at a high 0.5 C rate, and capacity retentionover 150 cycles with a Coulombic efficiency >99%. This exceptionalcapacity retention and stability may be attributable topolysulfide-attracting heteroatom functionalities present in thenanostructures due to incomplete carbonization.

Furthermore, sulfur-based cathodes of the present invention do notnecessarily need to contain additional binders, additives, or a currentcollector, components that can make up as much as 50% of the electrodeweight in the electrodes of the prior art. Therefore, in a more directcomparison with common slurry-based nanocomposites, the effectivecapacity of the sulfur-based cathode at 0.5 C can be translated to about997 mAh g⁻¹ over 150 cycles. The simplicity of the sulfur meltingprocedure and the exceptional cycling stability of these free-standingsulfur-based composites provide a practical means for manufacturing Li—Scathodes.

The present invention is considerably more efficient than commonly usedmethods for diffusing sulfur into porous electrically conductivesubstrates to produce useful electrode structures. These methodstypically take 2-48 hours, as well as other drawbacks. For example, thetemperatures required for vapor deposition techniques (i.e. >300° C.)requires significantly more energy input, use of inert gases to preventformation of toxic SO₂, and wastes a significant proportion of thesulfur raw material (about 60%) due to sublimation under the hightemperature conditions over the long time periods needed (in manyhours). The present invention can prepare high-capacity cathodes withinjust a few seconds at significantly lower temperatures (about 140° C.)with minimal waste of raw material. Further, the fast process and lowertemperature eliminates hazards of prior art methods, thereby allowingthe process to be performed in open air. Moreover, the prior art methodsare not suitable for large-scale production, further hindering their usein production of new types of sulfur-containing cathodes. The presentinvention can be easily implemented in an industrial setting forlarge-scale production since it lends itself to use in combination withcontinuous production methods like roll-pressing.

The cathodes of the present invention may be used in lithium-sulfur orsodium-sulfur batteries. Such lithium-sulfur batteries have highdischarge capacities in a range of from about 1200 mAh g⁻¹ to about 1675mAh g⁻¹, or preferably from about 1400 mAh g⁻¹ to about 1625 mAh g¹, ormore preferably from about 1500 mAh g⁻¹ to about 1600 mAh g⁻¹. Theselithium-sulfur batteries also have an energy density in a range of fromabout 1800 Wh kg⁻¹ to about 2500 Wh kg⁻¹, or preferably from about 2000Wh keto about 2400 Wh kg⁻¹, or more preferably from about 2200 Wh kg⁻¹to about 2300 Wh kg⁻¹.

An exemplary embodiment of the disclosure is set forth in details below.

A. EXPERIMENTAL PROCEDURE

1. Carbon Nanofibers

For the preparation of carbon nanofibers, polyacrylonitrile (PAN, Sigma,MW=150,000) and N,N-dimethyl formamide (DMF, Sigma) were used withoutfurther treatment. Solutions were prepared by mixing 10 wt. % PAN in DMFand stirring for a minimum of 4 hours. These solutions were placed in a5-mL syringe and a 20-gauge stainless steel needle. The flowrate usedfor electrospinning was 0.2-0.4 mL/hr with a constant applied voltage ofabout 9 kV. The distance between the nozzle tip and aluminum foilcollector was about 15 cm. Free-standing PAN fiber mats were then madefrom the electrospun fibers and stabilized at 280° C. in air for 5 hrusing a convection oven. Subsequent carbonization was carried out in atube furnace at 1000° C. for 1 hr at a heating rate of 2° C./min undernitrogen flow.

2. Cathode Fabrication

Commercial sulfur (Sigma, 100 mesh) was used as purchased withoutadditional treatment. CNF mats were punched into discs of about 11 mm indiameter. Sulfur powder was gently spread across the top of the CNF matsurface until sufficient mass was present to obtain a sulfur content ofabout 50-60 wt. % or about 1-2 mg cm⁻² in the S-CNF composite electrode.The sulfur and CNF mat were then placed between sheets of weighing paperand compressed in a heat press (Carver), pre-heated to 140° C., under amild pressure of about 200 psi for precisely 5 seconds.

3. Electrochemical Characterization

In an argon-filled glovebox, coin cells (CR2032) were assembled from theS-CNF electrode, Celgard separator, 30-40 μL of electrolyte, a lithiumfoil anode, and nickel-foam spacers. The S-CNF electrodes were useddirectly as cathodes in these coin cells without any further treatment.The S-CNF electrodes were positioned such that the top side throughwhich the sulfur was infused was facing away from the separator. Theelectrolyte composition was 1.85M lithium trifluoromethanesulfonate(Acros Organics) and 0.1M lithium nitrate (Sigma Aldrich) in a mixtureof 1,2-dimethoxyethane (Sigma Aldrich) and 1,3-dioxolane (AcrosOrganics) in a 1:1 volume ratio. Prior to electrochemical testing, cellswere allowed to rest at open circuit potential for 3 hours. Cyclabilitytesting was completed with a MACCOR (4000 series) battery cycler atvarious C-rates (with 1 C=1,675 mA g⁻¹) in a potential range of 1.8 Vand 2.6 V, with respect to Li/Li⁺. Cyclic voltammetry (scan rate of 0.05mV/s) and electrochemical impedance spectroscopy (frequency range of0.01 Hz to 100 kHz) were completed on a potentiostat (Gamry reference1000).

4. Materials Characterization

Nanofibers and S-CNF cathodes were characterized with a scanningelectron microscope (SEM, Zeiss Supra 50VP) equipped with EDS (Oxford)for carbon and sulfur mapping. The materials were characterized withX-ray photoelectron spectroscopy (XPS, Physical Electronics VersaProbe5000 spectrometer) with a monochromated Al Ka excitation source andX-ray diffraction (Rigaku SmartLab).

B. RESULTS AND DISCUSSION

Free-standing sulfur-carbon nanofiber (S-CNF) cathodes were fabricatedby diffusing melted sulfur into a carbon nanofiber mat produced byelectrospinning. The advantages of using electrospun nanofibers forrechargeable battery electrodes are two-fold: (i) they are inherentlyfree-standing at each step of fabrication obviating the need forbinders, additional or separate current collectors, or costlyslurry-based fabrication steps and (ii) CNFs provide an excellentnano-architecture for cathodes in the form of continuous conductivepathways for uninterrupted electron-transport. Furthermore, theinter-fiber macropores facilitate electrolyte diffusion to the sulfurfor redox reactions to occur.

Schematics of the S-CNF cathode fabrication and battery cellconfiguration are shown in FIGS. 1A-1B. The sulfur powder was disperseddirectly along one side of the CNF disc. After sulfurizing, theelemental sulfur was no longer visible to the naked eye (see FIG. 2).The sulfur was rapidly melted and diffused into the CNF mat in just 5seconds by applying a slight pressure of about 200 psi and heat in ahydraulic heat-press. The press plates were preheated to 140° C. torapidly melt the sulfur (melting temperature of sulfur is about 119°C.), but operated well below the temperature at which sulfurring-opening polymerization begins (159° C.). Unlike conventional sulfursublimation or melt-diffusion, this simplified method is compatible withroll-to-roll industrial processes as well as safe under normal operatingconditions in air without the risk of hazardous formation of hazardousreaction products, such as SO₂.

In the fabrication process, sulfur was infused from one side of the CNFmat during rapid heat treatment such that the sulfur diffused throughoutthe thickness of the CNF. In higher magnification SEM micrographs fromboth the side of sulfur-infusion (FIG. 3A) and opposite side (FIG. 3B)exhibited sulfur deposited between nanofibers on both sides of thecathode. FIGS. 3C-3E show the cross-sectional SEM micrograph andcorresponding sulfur and carbon EDS maps. The relatively uniform sulfurdiffusion throughout the CNF mat demonstrated that large inter-fiberspacing inherent to CNFs provided vast open pathways for easy diffusionof the molten sulfur throughout the CNF mat.

Thermogravimetric analysis confirmed the sulfur content in the S-CNF, asshown in FIG. 4. The TGA mass percent loss was in agreement withestimates based on the weights measured before and after heat treatment.Furthermore, rapid heat-press treatment did not alter the state of thesulfur as confirmed by XRD (see FIG. 5). The rapid melt techniquetherefor allows precise control of sulfur loading based solely on theamount of sulfur initially dispersed on the CNF mat prior to theheat-press treatment. High sulfur loadings can easily be achieved withthis technique, with sulfur loadings as high as 90% being easilyachieved.

S-CNFs were used directly in coin cells for electrochemicalcharacterization (FIG. 1B). In FIG. 6, cyclic voltammetry showedreduction and oxidation peaks characteristic of lithium-sulfur reactionsin a Li—S battery. During discharge, the two reduction peaks observed atabout 2.35V and about 2V correspond to S₈ reduction to higher orderpolysulfides and lower order polysulfides, respectively. Upon charging,the polysulfides undergo oxidation from insoluble, lower orderpolysulfides back to soluble, higher order polysulfides, and eventuallyS₈. During the first anodic scan, a sharp oxidation peak at about 2.45Vshowed a slight overpotential that decreases after subsequent cycles inwhich the peak broadens into two peaks between 2.3V and about 2.45V.This can be attributed to the rearrangement of sulfur during the firstdischarge-charge cycle to a more electrochemically favorable state.

Discharge-charge curves in FIG. 7 show high initial capacities of 1154mAh g⁻¹, 1022 mAh g⁻¹, and 845 mAh g⁻¹ for cathodes cycled at 0.1 C, 0.2C, and 0.5 C, respectively. The voltage plateaus in the discharge-chargecurves were in agreement with the lithium-sulfur redox voltages seenbefore, again confirming Li—S reactions were occurring. For S-CNF at 0.1C, there was much lower polarization. The charge and dischargecapacities were nearly equal, which indicates highly reversibleelectrochemical reactions (high Coulombic efficiency). See FIG. 8. TheCoulombic efficiency during the first cycle at higher C-rates decreasedand the upper reduction potential slightly increased, possibly due tokinetic limitations during the first reduction of S₈/high-orderpolysulfides. To stabilize cells for long-term cycling, an initialconditioning cycle of 0.1 C and/or 0.2 C was performed before the final0.2 C or 0.5 C rates, respectively.

At a standard rate of 0.2 C, a discharge capacity of 1057 mAh g⁻¹ wasobtained for S-CNF, as shown in FIG. 9. Following a similar trend as forthe above-characterized cathodes at 0.1 C, the discharge capacity fadedwithin 20 cycles to 665 mAh g⁻¹, and finally to 534 mAh g⁻¹ after 100cycles. In the first 20 cycles, the average capacity fade rate was 2.4%per cycle. After 20 cycles, the average capacity fade rate was 0.3% percycle, about an order of magnitude lower.

In an attempt to limit the large capacity drop to just a few cycles, theS-CNF cathodes were tested with a conditioning cycle, as shown in red inFIG. 10. This S-CNF cathode was conditioned at 0.1 C for the firstcycle, then 0.2 C for all subsequent cycles. The discharge capacity forconditioned cathode run at a rate of 0.2 C was just 755 mAh g⁻¹, but theconditioning provided a rapidly equilibrated discharge capacity within 3cycles. The discharge capacity stabilized to 584 mAh g⁻¹, beyond which93% of that capacity was retained after 100 cycles, with an averagecapacity fade of only 0.37% per cycle and >99% average Coulombicefficiency

The capabilities of the S-CNF cathode were further demonstrated at arate of 0.5 C with using similar conditioning methods. In FIG. 11, S-CNFcathodes were tested at rate of 0.5 C after either a preliminarysingle-step conditioning or a two-step conditioning involving (i) aninitial cycle at 0.2 C (black) and (ii) an additional primary cycle at0.1 C plus a secondary conditioning cycle at 0.2 C (red). The impact ofconditioning becomes apparent at faster discharge-charge rates, forwhich two-step conditioning stabilizes the cell almost immediately,whereas single-step conditioning still results in a slow, but relativelylarge capacity fade. For the single-step conditioning, the initial 0.5 Cdischarge capacity was 845 mAh g⁻¹. Within 20 cycles, the capacityretention was only 69% with a 93.4% Coulombic efficiency. After 100discharge-charge cycles, the discharge capacity was only 310 mAh g⁻¹,roughly a 37% retention.

With a two-step conditioning, S-CNF cathodes produced a dischargecapacity of 550 mAh g⁻¹ at 0.5 C with 100% retention and an averageCoulombic efficiency of >99% over 150 cycles. The low discharge capacityat the 0.5 C rate (<40% of the theoretical maximum) showed relativelypoor utilization of the sulfur, but provided excellent cycling stabilityover many cycles. With a >99% Coulombic efficiency and a steadilyincreased discharge capacity, the two-step conditioned S-CNF cathodecycled at 0.5 C showed an improvement in sulfur utilization over 150cycles. To better compare to the structures in literature in whichcurrent collectors are used, a calculation for the effective capacity(shown below), is also plotted in FIG. 12, and demonstrates thateffective capacity is nearly about 1000 mAh g⁻¹.

Effective Capacity Calculations

On average, sulfur composites cathodes are combined with additives havea resulting sulfur composition of 50-70 wt. % and about 1-2 mg cm⁻². Forcomparison, we assume 60% and 1.5 mg cm⁻² loading, thus the weight ofthe cathode (sulfur and additives) would be:

${{Cathode}\mspace{14mu}{weight}\text{:}\mspace{14mu}\frac{1.5\mspace{14mu}{mg}_{S\; 8}}{60\mspace{14mu}{wt}\mspace{14mu}\%}} = {2.5\mspace{20mu}{mg}_{cathode}}$With a conservative estimate of about 2 mg cm⁻² for the currentcollector, the total weight of cathode and current collector:2.5 mg_(cathode)+2.0 mg_(current collector)=4.5 mg_(total)If the capacity of such cathodes is about 1000 mAh g⁻¹ _((sulfur)), thecapacity equivalent {mAh g⁻¹ _((total=cathode+current collector))} basedon the cathode and current collector would be:

${\frac{1000\mspace{14mu}{mAh}}{g_{S\; 8}} \times \frac{1.5\mspace{14mu}{mg}_{S\; 8}}{4.5\mspace{14mu}{mg}_{total}}} = {333.3\mspace{14mu}{mAh}\mspace{14mu} g_{total}^{- 1}}$For the S-CNF cathodes of the present invention with 60 wt. % sulfur and1.5 mg cm⁻² loading, the only components are sulfur and CNF, whichserves as a current collector. The average discharge capacity based onsulfur mass is 550 mAh g⁻¹. Based on the same calculations shown above,the equivalent capacity would be:

${\frac{550\mspace{14mu}{mAh}}{g_{S\; 8}} \times \frac{1.5\mspace{14mu}{mg}_{S\; 8}}{2.5\mspace{14mu}{mg}_{total}}} = {330\mspace{14mu}{mAh}\mspace{14mu} g_{total}^{- 1}}$Thus, the effective capacity ratio comparing our cathodes to standard(slurry-based) cathodes would be about 1.8, as shown below:

${\frac{1000\mspace{14mu}{mAh}\mspace{14mu} g_{S\;{8@{std}}}^{- 1}}{550\mspace{14mu}{mAh}\mspace{14mu} g_{S\;{8@{SCNF}}}^{- 1}} \times \frac{330\mspace{14mu}{mAh}\mspace{14mu} g_{{total}@{SCNF}}^{- 1}}{333.3\mspace{14mu}{mAh}\mspace{14mu} g_{{total}@{std}}^{- 1}}} = {\sim 1.8}$Therefore, the effective discharge capacity for an S-CNF cathodeexhibiting 550 mAh g⁻¹ _(sulfur) would be about 991 mAh g⁻¹.

The initial drop in discharge capacity during the first 3 cycles can beattributed to large sulfur particles in which only the S₈ molecules onthe surface of the particles are accessible. The rapidly equilibrateddischarge capacity and excellent cycling stability demonstratepolysulfide blocking. It is believed that the highly stable performanceafter conditioning results from the CNF host, which provides both aphysical blocking mechanism as well as a chemical affinity mechanism topolysulfides. Previous works have demonstrated great enhancement inperformance by simply inserting interlayers, such the CNF, between thecathode and separator thereby creating a physical barrier.

To compare the performance of the S-CNF cathodes of the presentinvention with a majority of structures in the literature, thesimplified construction of the S-CNF cathodes must be taken intoaccount. As previously mentioned, the CNFs provide continuous conductivepathways, which allows both the binders and the heavy metal currentcollector to be eliminated. For example, the current collector for anaverage Li—S cathode with a 1.5 mg cm⁻² sulfur loading (about 60 wt. % Sin C/S cathode), would conservatively make up close to 50% of theelectrode weight. Therefore, the long-term cycling data for S-CNF testedat 0.5 C was translated into effective capacity for rational comparisonwith literature in which nanostructured cathodes undergo slurry-basedprocessing onto an additional current collector. Other cathodes thatrequire such construction are overlaid in FIG. 13 for direct comparisonwith the adjusted capacity estimate (all at 0.5 C rates). By aconservative estimate (see supporting calculation above), the effectivecapacity of the S-CNF was demonstrated to be 997 mAh g⁻¹ after 150cycles at 0.5 C. Not only does the literature require slurry-processingand current collectors, but also very long sulfur deposition times: e.g.a 20 hour melt-diffusion deposition, a 40 hour solution-baseddeposition, a 16 hour two-step melt-diffusion deposition, a 4 hoursolution-based deposition, and a 12 h melt-diffusion deposition. See Z.Li, X. Li, Y. Liao, X. Li and W. Li, Journal of Power Sources, 2016,334, 23-30; G. Li, J. Sun, W. Hou, S. Jiang, Y. Huang and J. Geng,Nature Communications, 2016, 7, 10601; N. W. Li, Y. X. Yin and Y. G.Guo, RSC Advances, 2016, 6, 617-622; and Z. Wei Seh, W. Li, J. J. Cha,G. Zheng, Y. Yang, M. T. McDowell, P. C. Hsu and Y. Cui, NatureCommunications, 2013, 4, 1331; and C. Zhang, H. B. Wu, C. Yuan, Z. Guoand X. W. Lou, Angewandte Chemie, 2012, 124, 9730-9733. Therefore, thesimplified nature of the S-CNF fabrication could be very important inpractical manufacturing of Li—S batteries especially when comparing thedata based on the elimination of the dead-weight of the currentcollector.

Moreover, studies using DFT calculations have shown that polysulfideshave a much greater affinity for functional groups rather than purecarbon. During PAN stabilization, the polymer chains undergo oxidationand cyclization reactions to form a ladder structure. Upon further heattreatment, deaminization of the ladder structure yields pure carbonizednanofibers. However, due to incomplete reactions, the CNF still containssome functional groups along the surface. Using XPS, the final CNFcomposition was determined to contain 89% carbon, 5% nitrogen, and 6%oxygen, as summarized in Table 1. The deconvoluted C1s, N1s, and O1speaks for CNFs are shown in FIGS. 14A-14C and the contributions fromeach functional species are summarized in Table 1.

TABLE 1 Summary of the high resolution XPS C 1s, N 1s, and O 1s spectraof the electrospun CNFs and composition (%). Carbon 89% eV % Nitrogen 5%eV % Oxygen 6% eV % C—C/C═C 284.6 56.5 C—N═C 398.3 31.1 O═R 531.1 53.5C═N 285.4 29.1 C—N═C/N—H 400.6 43.8 R—O—R 532.6 46.5 C—O 286.4 9.6Quaternary 401.6 25.1 C═O 287.5 3.1 O—C═O 289.0 1.7

Based on XPS, the CNF composition contains 89% carbon, 5% nitrogen, and6% oxygen. The deconvoluted C 1s, N 1s, and O 1s peaks for CNFs areshown in FIG. 14A-14C respectively and the contributions from eachfunctional species are summarized in Table 1. The functional groupspresent in the CNF samples might serve to attract polysulfides, forchemical adsorption on the non-porous CNF surface, which benefits thelong term reversibility of the S-CNF cathodes. Within the carbon portionof the CNF material, only about 57% is graphitic carbon (—C—C—/—C═C—,285 eV), meaning more than 40% of the carbon functional groups presenthave greater polysulfide interactions (i.e. polysulfide bindingenergy >0.55 eV). Moreover, numerous studies have shown the relativelyhigh chemical affinity of polysulfides to oxygen and nitrogen groups(among others) with respect to pure carbon. See H. J. Peng, T. Z. Hou,Q. Zhang, J. Q. Huang, X. B. Cheng, M. Q. Guo, Z. Yuan, L. Y. He and F.Wei, Advanced Materials Interfaces, 2014, 1; Q. Pang, X. Liang, C. Y.Kwok and L. F. Nazar, Journal of The Electrochemical Society, 2015, 162,A2567-A2576; L. Ma, H. L. Zhuang, S. Wei, K. E. Hendrickson, M. S. Kim,G. Cohn, R. G. Hennig and L. A. Archer, ACS Nano, 2016, 10, 1050-1059;J. Song, T. Xu, M. L. Gordin, P. Zhu, D. Lv, Y. B. Jiang, Y. Chen, Y.Duan and D. Wang, Advanced Functional Materials, 2014, 24, 1243-1250; K.A. See, Y. S. Jun, J. A. Gerbec, J. K. Sprafke, F. Wudl, G. D. Stuckyand R. Seshadri, ACS Applied Materials & Interfaces, 2014, 6,10908-10916; G. Zhou, L. C. Yin, D. W. Wang, L. Li, S. Pei, I. R.Gentle, F. Li and H. M. Cheng, ACS Nano, 2013, 7, 5367-5375. Forexample, about 75% of the nitrogen in the CNFs used on our workcorrespond to pyridine functionalities (about 398 and about 400 eV). SeeD. W. Wang, F. Li, L. C. Yin, X. Lu, Z.-G. Chen, I. R. Gentle, G. Q. Luand H. M. Cheng, Chemistry—A European Journal, 2012, 18, 5345-5351. DFTcalculations by Peng et al showed pyridine groups have twice the bindingenergy with soluble polysulfides as graphitic carbon, most likely due tothe lone electron pair that Li⁺ ions directly binding to theelectron-rich nitrogen atom. See H. J. Peng, T. Z. Hou, Q. Zhang, J. Q.Huang, X. B. Cheng, M. Q. Guo, Z. Yuan, L. Y. He and F. Wei, AdvancedMaterials Interfaces, 2014, 1. Moreover, oxygen functional groups showgreater affinity to polysulfides compared to graphitic carbons. See G.Zhou, L. C. Yin, D. W. Wang, L. Li, S. Pei, I. R. Gentle, F. Li and H.M. Cheng, ACS Nano, 2013, 7, 5367-5375; and H. J. Peng, J. Q. Huang, M.Q. Zhao, Q. Zhang, X. B. Cheng, X. Y. Liu, W. Z. Qian and F. Wei,Advanced Functional Materials, 2014, 24, 2772-2781.

Coin cell batteries with the S-CNF cathode were assembled in reverseorder (sulfur-infusion side facing the separator) to verify the impactof the side-of-sulfur-infusion and a slight decrease in performance wasfound (see FIG. 15) which could be attributed to slightly denser sulfuron the side-of-infusion that is not detectable by low-resolution EDSmapping. This could be expected due to the nature of the large sulfurparticles and the short melting time such that larger sulfur particlescould not fully melt and diffuse into the CNF during the quick process.

In addition, the effect of asymmetric heating was also investigated totry to induce a pronounced sulfur gradient. The sulfur-infused sideplate was heated to the standard 140° C. and the opposing plate to only120° C. (anything below 120° C. for the opposite plate would not meltthe sulfur within 5 seconds). It was found this merely decreased theoverall performance most likely due to insufficient melt diffusion ofsulfur particles during processing (see FIGS. 16A-16C and 17A-17C).

Furthermore, the effect of increasing the heating time from 5 second to15 and 30 seconds was investigated. SEM and EDS showed that increasingthe heating time to 15 seconds induced formation of larger sulfuragglomerates. Upon further increasing the heating time to 30 seconds,the formed sulfur agglomerates also increased in size and number (seeFIGS. 18A-18C and 19A-19 C). The cycling performance decreasedconsiderably with 15 seconds of heat pressing, (see FIG. 20) thusvalidating that the rapid 5 second melt pressing was best for diffusingthe commercial sulfur into the CNF mat without significant sulfuragglomeration.

Being completely free-standing, S-CNF cathodes eliminate the problem ofsignificant excess weight being added to the cathodes by obviating theneed for insulating binders and a separate current collector. Table 2shows a direct comparison with literature. Most “free-standing” studiesrequire vacuum infiltration (not suitable for scale-up) as well as longduration sulfur-deposition methods that can only be done in smallbatches within a furnace or oven (also not suitable for scale-up). A fewmethods utilize electrospun carbon nanofibers as the host material forsulfur composite cathodes but such known methods either includeadditional processing, additives or provide a relatively low C-rateperformance.

The only method that used a truly short sulfur-infusion period requiredCNT-growth on a carbon cloth substrate (additional current collector)that contributed up to 37% of the discharge capacity. Moreover, the CNFsof the present invention are significantly less expensive than CNTs,even on a commercial scale. In addition, the performance of the CNTcathodes was only demonstrated over a very short number of cycles at lowC-rates.

TABLE 2 Comparison of S-CNF to relevant Li—S cathodes in literature -cathode material, sulfur deposition technique, sulfur loading, dischargecapacity, and retention Discharge Capacity % Retention Cathode CompositeDeposition^(a) Loading^(b) (C-rate) (# of cycles) S-MWCNT (slurry-based)[⁴⁹] MD 24 h ~56%  866 mAh g⁻¹ (0.5 C) 70 (50) MWCNT/S vacuum filtered[²⁰] MD 2.5 h 54% 995 mAh g⁻¹ (0.05 C) 70 (150) Graphene oxide/ZnOadditive [²⁹] MD 10 h ~1 mg cm⁻² ~1000 mAh g⁻¹ (0.2 C) 85 (100) 796 mAhg⁻¹ (1 C) 81 (250) Mesoporous graphene oxide paper [²⁴] VD/MD 14 h 55%1393 mAh g⁻¹ (0.1 C) 49 (50) Few-layer graphene sheets [⁵⁰] Solution 52%800 mAh g⁻¹ (0.5 C) 65 (50) 403 mAh g⁻¹ (2 C) 74 (400) Grapheneoxide/nano sulfur freeze- MD 5 h 63% 957 mAh g⁻¹ (0.2 C) 84 (200) driedsuspension [⁵¹] 657 mAh g⁻¹ (1 C) 90 (168) TiO₂-grafted carbonizedfilter paper MD 12 h ~2 mg cm⁻² 1606 mAh g⁻¹ (0.5 C) 53 (200)(free-standing) [²⁵] 1200 mAh g⁻¹ (1 C) 45 (500) Electrospun porous CNF(free- VD 12 h 72% 1214 mAh g⁻¹ (0.2 C) 78 (200) standing) + graphene[¹²]* KOH-activated, electrospun MD 8 h 52% 900 mAh g⁻¹ (0.03 C) 76(100) CNF/Cu (free-standing) [⁵²]* KOH-activated, electrospun MD 8 h 40%1592 mAh g⁻¹ (0.03 C) 40 (100) CNT/CNF (free-standing) [²⁶]* ElectrospunCNF (free-standing) [³¹]* VD 15 m ~55%  1250 mAh g⁻¹ (0.06 C) 76 (50)CVD-grown CNTs on carbon MD 2 m <45%^(c)  ~1175 mAh g⁻¹ (0.04 C) 77 (10)substrate [³²] ~800 mAh g⁻¹ (0.17 C) 100 (20) This invention:electrospun CNF UR 5 s ~55%  755 mAh g⁻¹ (0.2 C) 72 (100)(free-standing) 550 mAh g⁻⁻¹ (0.5 C) 100 (150) ^(a)Deposition techniquesinclude melt diffusion (MD), vapor deposition (VD), and the depositiontechnique of the present invention (ultra-rapid, UR) ^(b)Loading basedon all components from reported values and/or the current collectorweight ^(c)The bare carbon cloth substrate (without CNTs) contributed36% of capacity reported, which has a high specific weight of about 8.5mg cm⁻² thus reducing the sulfur content in the final electrodes to <45%*Free-standing electrodes fabricated by electrospinning

C. CONCLUSION

The present invention provides an ultra-rapid method for diffusingsulfur into nanostructures such as electrospun carbon nanofibers to beused cathodes for lithium-sulfur batteries. The sulfur deposition methoddrastically reduced the time to infuse the cathode with sulfur fromabout 10 hours to only 5 seconds, showing great promise towardspractical fabrication of Li—S cathodes. In just 5 seconds, CNF weresulfurized with up to 60 wt. % sulfur and produced highly reversibleelectrochemistry. S-CNF cathodes demonstrated 100% capacity retention atover 150 cycles at 0.5 C (discharge capacity of about 550 mAh g⁻¹). Thegood performance of such simple cathodes suggests that chemicaladsorption may play a role for reversibility and capacity retention asthe role of micro-/meso-pores for complete sulfur confinement. Thepolysulfide-CNF interactions are of interest. Moreover, the electrospunCNF structures have two important elements that allow such simplecathodes:

(i) they are inherently free-standing thereby obviating the need forbinders, additional current collectors, or costly slurry-basedfabrication. The weight contribution of these inactive cell componentsin prior art electrodes is significant and estimated to contribute 30-50wt. % of the entire cathode. See N. Li, Z. Chen, W. Ren, F. Li and H. M.Cheng, Proceedings of the National Academy of Sciences of the UnitedStates of America, 2012, 109, 17360-17365. On the industrial level,electrode processing (i.e. slurry mixing and casting) makes up 30% ofthe manufacturing costs in commercialized lithium-ion batteries. See ref53.

(ii) CNFs provide excellent nano-architecture for cathodes viacontinuous conductive pathways for uninterrupted electron-transport andthe inter-fiber macropores facilitate electrolyte diffusion to thesulfur for redox reactions to occur. In addition, the relatively simplenanoarchitecture may shed light on the importance of the role ofmesopores (sulfur entrapment) with respect to sulfide chemisorption.

Sulfide adsorption mechanisms may also play a role in the fabricationprocess. The present method allows improvement of the cathode withsmaller sulfur (nano-sulfur) to improve sulfur utilization andaccessibility to provide ultra-high sulfur loading (>8 mg cm⁻², >80 wt.%), and tailoring the CNF materials to improve shuttling and utilizationwithout compromising the simplicity of the method. The simplefabrication of these binder-free, free-standing cathodes presents apractical substitute for cathode preparation.

All documents mentioned herein are hereby incorporated by reference intheir entirety or alternatively to provide the disclosure for which theywere specifically relied upon. The applicant(s) do not intend todedicate any disclosed embodiments to the public, and to the extent anydisclosed modifications or alterations may not literally fall within thescope of the claims, they are considered to be part hereof under thedoctrine of equivalents.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meanings of the terms inwhich the appended claims are expressed.

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What is claimed is:
 1. A free-standing electrically conductive porousstructure, consisting essentially of: an electrically conductive poroussubstrate with a layer of sulfur on a surface of the electricallyconductive porous substrate, wherein the sulfur layer includes anadditive to reduce the viscosity of melted sulfur for diffusing themelted sulfur into the substrate; wherein at least a portion of thelayer of sulfur is in pores of the substrate and located on a surface ofthe pores of the substrate, the free-standing electrically conductiveporous structure contains at least 50 wt.% of sulfur and less than 10wt.% of graphene, both based on a total weight of the electricallyconductive porous structure, the additive to reduce the viscosity ofmelted sulfur for diffusing the sulfur into the substrate comprises anelement selected from the group consisting of selenium, tellurium,bromine and iodine, the sulfur layer contains less than 30 wt.% of acombination of the additive to reduce the viscosity of melted sulfur fordiffusing the sulfur into the substrate and one or more optionaladditives selected from a conductive additive and an additive thatprevent or reduces active material dissolution or loss into anelectrolyte during device operation, and when the porous structure isconfigured as a cathode for a battery, the battery including the cathodehas a C rate of at least 0.2 C at a discharge capacity of from about 400mAh g⁻¹ to about 1675 mAh g⁻¹ with a cycling stability of at least 100cycles.
 2. The free-standing electrically conductive porous structure ofclaim 1 wherein the electrically conductive porous structure has asulfur loading of at least about 1.0 mg cm⁻².
 3. The free-standingelectrically conductive porous structure of claim 1, wherein the C rateis at least 0.5 C and the cycling stability is at least 120 cycles. 4.The free-standing electrically conductive porous structure of claim 1,wherein the free-standing electrically conductive porous structure has aflexibility that passes a Mandrel Bend test of ASTM D522.
 5. Thefree-standing electrically conductive porous structure of claim 1,wherein the discharge capacity is in a range of from about 700 mAh g⁻¹to about 1625 mAh g⁻¹.
 6. The free-standing electrically conductiveporous structure of claim 1, wherein the sulfur layer comprises theconductive additive and the conductive additive is selected from thegroup consisting of conductive carbon powders, graphite powders,mesoporous carbons, activated carbons, carbon nanotubes, MXenes,conductive polymers, conductive metal oxides/suboxides, metals and anyother material that conducts electrons.
 7. The free-standingelectrically conductive porous structure of claim 1, wherein theelectrically conductive porous substrate comprises a material selectedfrom carbon nanofibers, carbon nanotubes, carbon rods, and combinationsthereof.
 8. The free-standing electrically conductive porous structureof claim 1, wherein the electrically conductive porous substrate has aporosity in a range of from about 10% to about 90% and a conductivity ina range of from about 10⁻³ S/cm to about 10⁵ S/cm.
 9. The free-standingelectrically conductive porous structure of claim 1, wherein the sulfurlayer comprises the additive that prevents or reduces active materialdissolution or loss into an electrolyte during device operation and theadditive that prevents or reduces active material dissolution or lossinto an electrolyte during device operation comprises an additive thatinteracts with sulfur or polysulfides by polar interaction or Lewis-acidbase interaction, said additive being selected from the group consistingof compounds containing polar elements, polymers and compoundscontaining metals such as titanium or vanadium.
 10. The free-standingelectrically conductive porous structure of claim 1, wherein theelectrically conductive porous structure has a sulfur content in a rangeof from about 50 wt.% to about 90 wt.%, based on a total weight of thestructure.
 11. The free-standing electrically conductive porousstructure of claim 1, wherein the sulfur in the electrically conductiveporous structure has an average particle size in a range of from 10 nmto 1000 nm.
 12. A cathode of a battery comprising the electricallyconductive porous structure of claim
 1. 13. The cathode of claim 12,wherein the cathode has a sulfur loading in a range of from about 0.1 mgcm⁻² to about 15 mg cm⁻².
 14. The cathode of claim 12, wherein thecathode does not include a separate current collector.
 15. Thefree-standing electrically conductive porous structure of claim 1,wherein the electrically conductive porous structure has a sulfurcontent in a range of from about 50 wt.% to about 80 wt.%, based on atotal weight of the structure.
 16. The free-standing electricallyconductive porous structure of claim 1, wherein the sulfur layer isapplied by a method comprising steps of: applying sulfur in powder orparticle form to the electrically conductive substrate; and heating thepowder form of the sulfur to a temperature of from about 119° C. toabout 170° C. with the electrically conductive porous substrate under apressure of from about 100 psi to about 2000 psi.
 17. The free-standingelectrically conductive porous structure of claim 16, wherein theheating step is carried out for a period of from 5 seconds to about 100seconds.
 18. The free-standing electrically conductive porous structureof claim 17, wherein the electrically conductive porous substrate is acarbon nanofiber mat and the heating step is carried out byroll-pressing or by using press plates.
 19. The free-standingelectrically conductive porous structure of claim 17, wherein the sulfurin powder form or particle form has a particle size not greater than 100mesh.
 20. The free-standing electrically conductive porous structure ofclaim 17, wherein the porous substrate has a porosity of at least about50%.
 21. The free-standing electrically conductive porous structure ofclaim 1, wherein the sulfur layer comprises up to 10 wt.% of the one ormore additives selected from a conductive additive, the additive toreduce the viscosity of melted sulfur for diffusing the sulfur into thesubstrate and the additive that prevents or reduces active materialdissolution or loss into an electrolyte during device operation.
 22. Thefree-standing electrically conductive porous structure of claim 1,wherein the sulfur layer comprises the conductive additive.